This application is for continued support of our project entitled "Electrophysiological and Clinical Evaluation of Bundle Branch Block - A Prospective Study" which was funded initially by Program Project Grant HL06336, and is a continuation of application HL21527, entitled "Prospective Study of Bundle Branch Block". Our primary goals are to define patients at high risk (from those who have bilateral bundle branch disease and intact atrioventricular conduction) of heart block, Stokes-Adams syncopal episodes, and sudden death. Specifically, we aim to determine the incidence of syncope, heart block and sudden death in patients with bi- and trifascicular conduction system diseases; the cause of death; significance of associated cardiovascular disease; the value of electrophysiological studies and of multivariate analysis. Once high-risk subgroups are identified, the value of prophylactic pacemakers in preventing syncopal episodes and sudden death will be evaluated by a prospective randomized study. However, this is a long-term goal and when such a study is to be initiated a separate protocol will be developed and submitted.